Antispam module's check_username_for_spam raises a TypeError when a user has no display name

[squahtx]

Describe the bug
The Synapse documentation incorrectly claims that the check_username_for_spam callback receives a userprofile that is a Dict[str, str] (synapse#12245). In practice, there can be Nones in the userprofile dict, eg.

{
    avatar_url: None, 
    display_name: None, 
    user_id: '@[redacted]:matrix.org'
}

This causes the antispam callback to raise an error:

TypeError: expected string or bytes-like object
  File "synapse/http/server.py", line 268, in _async_render_wrapper
    callback_return = await self._async_render(request)
  File "synapse/http/server.py", line 470, in _async_render
    callback_return = await raw_callback_return
  File "synapse/rest/client/user_directory.py", line 75, in on_POST
    user_id, search_term, limit
  File "synapse/handlers/user_directory.py", line 103, in search_users
    if not await self.spam_checker.check_username_for_spam(user):
  File "synapse/events/spamcheck.py", line 404, in check_username_for_spam
    if await callback(user_profile.copy()):
  File "synapse/events/spamcheck.py", line 152, in run
    return maybe_awaitable(wrapped_func(*args, **kwargs))
  File "mjolnir/antispam.py", line 129, in check_username_for_spam
    user_profile["display_name"]
  File "mjolnir/antispam.py", line 60, in is_user_banned
    if rule.matches(user_id):
  File "mjolnir/list_rule.py", line 54, in matches
    return self.regex.match(victim)

https://sentry.matrix.org/sentry/synapse-matrixorg/issues/243697/

[Yoric]

@Gnuxie Could you take a look?